Hydrogenation of aromatic nitrocompounds to aromatic amines

ABSTRACT

A method for the hydrogenation of aromatic nitrocompounds to aromatic amines, which comprises mixing an aromatic nitrocompound, hydrogen gas and a solvent together under elevated pressure and temperature to form a homogenous mixture in a supercritical or near-critical state and bringing the resulting homogenous mixture into contact with a catalyst to form the aromatic amine product.

This application is the national phase under 35 U.S.C. §371 of prior PCTInternational Application No. PCT/FI97/00099 which has an Internationalfiling date of Feb. 18, 1997 which designated the United States ofAmerica, the entire contents of which are hereby incorporated byreference.

TECHNICAL TECHNICAL FIELD OF THE INVENTION

The present invention concerns a method for the hydrogenation ofaromatic nitrocompounds to aromatic amines.

BACKGROUND OF THE INVENTION

Aromatic nitrocompounds are compounds where one or more nitro-groups arebound to a carbon atom in an aromatic ring. Typical aromaticnitrocompounds are e.g. nitrobenzene, nitrotoluene, nitroxylene anddinitrotoluene. Aromatic nitrocompounds can be hydrogenated tocorrespnding aromatic amines.

It is well known that many of the relatively high boiling aromaticamines can be prepared by liquid-phase hydrogenation of thecorresponding aromatic nitrocompound in the presence of a catalyst,either with or without the use of a solvent¹. Early catalytichydrogenations were performed in stirred batch reactors. In this knownmethod the aromatic nitrocompound is first dissolved into a suitablesolvent and pumped into a batch reactor containing the catalyst. Thereactor which is equipped with a stirrer is pressurized by hydrogen gas.After the reaction is completed, the product can be separated from thesolvent.

More recently batch processes have been replaced by continuousprocesses. A continuous liquid-phase process is illustrated by theprocess for diaminotoluene²,3. According to the process, dinitrotoluene(DNT) is catalytically hydrogenated at 150-200 bar and about 100° C. Asolution, which contains about 25 wt-% dinitrotoluene dissolved inmethanol, hydrogen and Raney-nickel, is pumped through a series ofreactors. Reactors are equipped with internal circulation to make thereaction more complete. After the reaction is completed, the pressure isreduced and the excess hydrogen removed in a gas-liquid separator andrecycled to the beginning of the process. Catalyst is then removed andrecycled, after which the solvent is distilled and recycled. The wateris removed in the dehydration column. The product purity is more than99-% pure diaminotoluene.

Low molecular weight alcohols, particularly methanol or ethanol, are themost commonly used hydrogenation solvents for hydrogenation of aromaticnitrocompounds. Other solvents, like acetic acid, ammonia, benzene,glycerol ethylene glycol, hydrochloric acid, sulfuric acid or water canbe used.

It is well known that the solubility of hydrogen into liquid-likesolvents is extremely small. Due to the this low hydrogen solubility andslow transfer to the catalyst surface where the reaction product isformed, the reaction in liquid-phase is relatively slow, and continuousreactors could not always be used effectively. Several mechanicalsolutions have been introduced. One solution is to use a spray columnwhere the solvent, nitroaromatic compound and catalyst are sprayed withthe hydrogen gas.

It is known that aromatic nitrocompounds can be hydrogenated in thevapor phase. In this method a vaporized nitroaromatic compound andhydrogen gas flow through the catalyst bed forming the product. Thereaction is usually very fast and high conversions are obtained with asingle pass. This method is continuous and in principle simple. A methodfor producing aniline from nitrobenzene in a trickle-bed reactor isdescribed in the literature⁴. Nitrobenzene is first vaporized and mixedwith a 200% excess of hydrogen gas. The hot gaseous mixture flows upwardinto the reduction chamber containing the copper-silica catalyst. Thereaction is very fast at 270° C. and 2-3 bar. After leaving the reactor,the reaction mixture containing aniline, hydrogen and water is cooled.Excess hydrogen is first removed after which water and aniline areseparated. Finally aniline is purified to a 99% product.

The catalytic hydrogenation of aromatic nitrocompounds to aromaticamines can be done in the vapor phase, provided that he boiling point ofthe compound is low enough and the starting material and the product arethermally stabile. These limitations mean that only relatively simplealiphatic and aromatic nitrocompounds, such as nitrobenzene andnitroxylene, can be hydrogenated in the vapor phase.

SUMMARY OF THE INVENTION

The present invention is based upon the discovery that aromaticnitrocompounds can be hydrogenated in the presence of a hydrogenatingcatalyst in a solvent, which is in the supercritical or near-criticalstate. The aromatic nitrocompound and hydrogen gas are dissolved intothe supercritical or near-critical state solvent. The resultinghomogenous mixture which is in the supercritical or near-critical stateis brought into contact with the catalyst, and the reaction product,i,e., the aromatic amine, is formed. The hydrogen concentration of thehomogenous mixture can be chosen freely and thus is not a restrictingfactor in the reaction. With the assistance of the present invention adecisive improvement on previously mentioned process limitations in theprior art can be obtained.

The word supercritical refers to the state of the solvent. For examplecarbon dioxide is in a supercitical state when its temperature is above31.1° C. and simultaneously the pressure is above 73.8 bar.Corresponding values for ethane are 32.4° C. and 48 bar, and for propane96.8° C. and 42 bar. Supercritical fluids exhibit both liquid- andgas-like properties, such as liquid-like density and gas-like viscosity.The diffusivity of supercritical fluids is between the values of gasesand liquids. Gas-like properties are considered to be beneficial inreaction chemistry due to enhanced mass transfer.

A particularly important property of supercritical fluids is, that theyare almost completely misciple with all kind of gases, includinghydrogen gas. This means that when the solvent is in a supercriticalstate the hydrogen gas needed to reduce the aromatic nitrocompound toaromatic amines can be easily mixed with the solvent. In the reactorswhere the reaction is carried out in a vapor phase, the nitroaromaticcompound is vaporized and mixed with hydrogen gas, which efficientlyremoves the mass transfer limitations encountered in liquid phasesystems.

However, relatively high temperatures, i.e. 300-475° C., are oftenneeded in the vapor phase systems. According to the present inventioncomplete miscibility of the nitroaromatic compound and hydrogen isobtained at significantly lower temperatures, i.e. 30-150° C.

The present invention makes possible the construction of more simple andmore efficient reactors for nitroaromatic amine production. Applying thesupercritical solvent can also simplify the overall process, for examplethe separation and purification of the product. The supercriticalsolvent, being usually a pressurized gas, can be relatively easilyseparated from the product by simply depressurising the solvent mixture.The reaction can be carried out in batch reactors, but obviouslycontinuous reactor systems are preferable in industrial practice.

Carbon dioxide, which is environmentally acceptable, non-toxic,non-flammable, relatively inexpensive, non-corrosive and easilyavailable can be utilized in the present invention. Carbon dioxide isused in fire fighting to extinguish the flames. Carbon dioxide can actboth as a solvent and as a safe gas in the hydrogenation processes. Lowmolecular weight hydrocarbons can also be used, such as, for example,ethane and propane, which are chemically stable against hydrogenation,which means that higher hydrogenation temperatures than in the case ofcarbon dioxide are required.

It is well known that the character of supercritical solvent can beenhanced by addition of a modifier like short chain alcohols or esters.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is described in detail in connection with thefollowing examples which are given as exemplary and not limitative ofthe present invention.

EXAMPLE 1 Reduction of methyl-p-nitrobenzoate in Carbon Dioxide withPd-Containing Catalyst

0.5 g methyl-p-nitrobenzoate (mp. 94-96° C.) and 0.057 g catalyst wereweighted and placed into the batch reactor (40 mL), after which thereactor was closed. The catalyst used was palladium on polyolefinfiber.The batch reactor was purged with carbon dioxide to remove the entrappedair, and was heated to 42° C. The system was then charged with carbondioxide and hydrogen gases, so that the total pressure was 180 bar andhydrogen partial pressure was 15 bar. The pressurized reaction mixturewas magnetically mixed for 30 minutes, after which the mixer was turnedoff and the reaction vessel was rapidly cooled and depressurized. Thecatalyst was then separated from the product. The product was analyzedwith Thin Layer Chromatography (TLC) and with liquid chromatography, andwas noted to be almost pure methyl-p-aminobenzoate. The purity was over95%.

EXAMPLE 2 Reduction of methyl-p-nitrobenzoate in Carbon Dioxide withNi-Containing Catalyst

Using procedures similar to example 1., methyl-p-nitrobenzoate washydrogenated. The catalyst was nickel on carbon and the reactiontemperature was 100° C. The total pressure was 80 bar and the hydrogenpartial pressure was 10 bar. The product was analyzed and purity ofmethyl-p-aminobenzoate was over 70%.

EXAMPLE 3 Reduction of methyl-p-nitrobenzoate in Carbon Dioxide withPt-Containing Catalyst

Using procedures similar to example 1., methyl-p-nitrobenzoate washydrogenated. The catalyst was platinum on activated carbon and thereaction temperature was 35° C. A small amount of methanol was used as amodifier. The total pressure was 250 bar and the hydrogen partialpressure was 40 bar. The product was analyzed and purity ofmethyl-p-aminobenzoate was over 90%.

EXAMPLE 4 Reduction of methyl-p-nitrobenzoate in Carbon Dioxide withCuO/CrO-Containing Catalyst

Using procedures similar to example 1., methyl-p-nitrobenzoate washydrogenated. The catalyst contained copperoxide/chromiumoxide. Thereaction temperate was 150° C., total pressure was 300 bar and thehydrogen partial pressure was 20 bar. The product was analyzed andpurity of methyl-p-aminobenzoate was over 60%.

EXAMPLE 5 Reduction of nitrobenzene in Carbon Dioxide with Pd-ContainingCatalyst

0.5 g nitrobenzene (mp. 5-7° C.) and 0.051 g catalyst were weighted andplaced into the batch reactor (40 mL), after which the reactor wasclosed. The catalyst used was palladium on polyolefinfiber. The batchreactor was purged with carbon dioxide to remove the entrapped air, andwas heated to 40° C. The system was then charged with carbon dioxide andhydrogen gases, so that the total pressure was 200 bar and hydrogenpartial pressure was 20 bar. Pressurized reaction mixture wasmagnetically mixed for 30 minutes, after which the mixer was turned offand the reaction vessel was rapidly cooled and depressurized. Thecatalyst was then separated from the product. The product was analyzedwith Thin Layer Chromatography (TLC) and with liquid chromatography, andwas noted to be over 90% pure aniline.

EXAMPLE 6 Reduction of 2,4-dinitrotoluene in Carbon Dioxide withPd-Containing Catalyst

0.5 g 2,4-dinitrotoluene (mp. 67-70° C.) and 0.27 g catalyst wereweighted and placed into the batch reactor (40 mL), after which thereactor was closed. The catalyst used was palladium on polyolefinfiber.The batch reactor was purged with carbon dioxide to remove the entrappedair, and was heated to 42° C. The system was then charged with carbondioxide and hydrogen gases, so that the total pressure was 200 bar andhydrogen partial pressure was 19 bar. Pressurized reaction mixture wasmagnetically mixed for 30 minutes, after which the mixer was turned offand the reaction vessel was rapidly cooled and depressurized. Thecatalyst was then separated from the product. The product was analyzedwith Thin Layer Chromatography (TLC) and with liquid chromatography, andwas noted to be over 90% pure 2,4-diaminotoluene.

EXAMPLE 7 Reduction of 2,4-dinitrotoluene in Carbon Dioxide withNi-Containing Catalyst

Using procedures similar to example 1., methyl-p-nitrobenzoate washydrogenated. The catalyst contained nickel on activated carbon. Thetemperature was 80° C. Total pressure was 250 bar and hydrogen partialpressure was 30 bar. The product was analyzed with Thin LayerChromatography (TLC) and with liquid chromatography, and was noted to beover 85% pure 2,4-diaminotoluene.

EXAMPLE 8 Reduction of methyl-p-nitrobenzoate in Propane withPd-Containing Catalyst

0.5 g methyl-p-nitrobenzoate and 0.2 g catalyst are weighted and placedinto a batch reactor (40 mL), after which the reactor is closed. Thecatalyst is palladium on carbon. The batch reactor is purged withpropane to remove the entrapped air, and is heated to 105° C. The systemis then charged with propane and hydrogen gases, so that the totalpressure is 75 bar and hydrogen partial pressure is 20 bar. Pressurizedreaction mixture is magnetically mixed for 30 minutes, after which themixer is turned off and the reaction vessel is rapidly cooled anddepressurized. The catalyst is then separated from the product. Themethyl-p-aminobenzoate product is analyzed with Thin LayerChromatography (TLC) and with liquid chromatography.

EXAMPLE 9 Reduction of methyl-p-nitrobenzoate in Ethane withPd-Containing Catalyst

0.5 g methyl-p-nitrobenzoate and 0.2 g catalyst are weighted and placedinto a batch reactor (40 mL), after which the reactor is closed. Thecatalyst is palladium on carbon. The batch reactor is purged with ethaneto remove the entrapped air, and is heated to 40° C. The system is thencharged with ethane and hydrogen gases, so that the total pressure is 90bar and hydrogen partial pressure is 20 bar. Pressurized reactionmixture is magnetically mixed for 30 minutes, after which the mixer isturned off and the reaction vessel is rapidly cooled and depressurized.The catalyst is then separated from the product. Themethyl-p-aminobenzoate product is analyzed with Thin LayerChromatography (TLC) and with liquid chromatography.

This invention has above been illustrated by referring to certainfavorable examples. It has not been intended to limit the scope of thisinvention to the above examples. Several modifications are possible,including stating materials, products, catalyst, pressure, temperature,time or operating mode, i.e. batch or continuous.

Literature Cited

1. J. I. Kroschwitz, M. Howe-Grant, Eds.; Kirk-Othmer, Encyclopedia ofChemical Technology, 4^(th) ed., vol 2, page 489.

2. H. Dierichs and H. Holzrichter, U.S. Pat. No. 3,032,586 (Apr. 18,1957)

3. H. Dierichs and H. Hoizrichter, Brit. Pat. 768 111 (Feb. 13, 1957).

4. O. C. Karkalits, Jr., C. M. Vanderwaart and F. H. Megson; U.S. Pat.No. 2,891,094 (Jun. 16, 1959)

What is claimed is:
 1. A method for the hydrogenation of aromaticnitrocompounds to aromatic amines, which comprises mixingan aromaticnitrocompound, hydrogen gas and a solvent together under elevatedpressure and temperature to form a homogenous mixture in a supercriticalor near-critical state and bringing the resulting homogenous mixtureinto contact with a catalyst to form the aromatic amine product.
 2. Themethod according to claim 1, wherein the solvent is selected from thegroup consisting of carbon dioxide, ethane, propane or a binary orternary mixture thereof.
 3. The method according to claim 1, wherein thecatalyst contains palladium-, platinum-, copper-, chromium- or nickel.4. The method according to claim 1, wherein methanol is added as amodifier.
 5. The method of claim 1, wherein the aromatic nitrocompoundis methyl-p-nitrobenzoate, 2,4-dinitrotoluene or nitrobenzene and thearomatic amine is methyl-p-aminobenzoate, 2,4-diaminotoluene or aniline.6. The method of claim 1, wherein the catalyst is selected from thegroup consisting of palladium/carbon, palladium/polyolefin fiber,platinum/carbon, nickel/carbon and copper oxide/chromium oxide.
 7. Themethod of claim 1, wherein the solvent is in said supercritical or nearcritical state.